There has been known a process comprising flocculating, using a flocculant, a paint component that is contained in a waste liquid produced by cleaning an aqueous paint, and separating the flocculated matter from the waste liquid by filtration through a nonwoven fabric (Japanese Patent Laid-Open No. 2001-149948, “Patent Document 1”).
On the other hand, there has also been known a process for treating organic waste, comprising a hydro thermal reaction treatment step of converting organic waste into a liquid by hydrothermal reaction under subcritical water conditions or supercritical water conditions, and an air oxidation treatment step of oxidizing the liquid produced by the hydrothermal reaction treatment with air. Furthermore, there has also been known a process for recovering methane gas, comprising the hydrothermal reaction treatment step or air oxidation treatment step, and a subsequent step of fermenting methane in the presence of sludge containing an anaerobic microorganism (Japanese Patent Laid-Open No. 2002-102870, “Patent Document 2”).
In the case of the process described in Patent Document 1, wastewater filtered through a nonwoven fabric can be disposed in any place, and the nonwoven fabric and the flocculated matter can be disposed as industrial waste. However, the wastewater filtered through the nonwoven fabric contains a remaining component that cannot be flocculated by a flocculant, for example, a neutralizer component (e.g., an amine, acid), a curing agent component (e.g., a melamine curing agent), an organic solvent component (e.g., an alcoholic solvent), a resin component (e.g., a low molecular weight acrylic resin and a low molecular weight polyester resin), or another additive component. Accordingly, such wastewater cannot be disposed in rivers or the like, because the wastewater pollutes the environment, for example, if disposed there, disadvantageously.
It is also possible to use a process comprising removing an organic substance that causes the environmental pollution as described above from wastewater containing the organic substance by biological treatment with bacteria. However, in such biological treatment, it is difficult to remove by separation or decompose a low molecular weight component (molecular weight: about 1,000 or less) such as an amine or a melamine curing agent.
The process described in Patent Document 2 treats an organic substance directly by hydrothermal reaction under supercritical conditions. Thus, the process exhibits low treatment efficiency, and poorly decomposes a low molecular weight organic substance such as a melamine curing agent, amine or organic solvent, for example, disadvantageously when treating a waste liquid containing a solid such as sludge. Furthermore, it is difficult even for the biological treatment to fully decompose and remove the low molecular weight organic substance remaining after the supercritical treatment.
An object of the present invention is to provide a process for treating wastewater which can efficiently separate and remove a low molecular weight organic substance that cannot be separated and removed in a conventional process and which provides wastewater containing only a small amount of a toxic substance.